1. Field of the Invention
This invention relates to solder mask coatings, and, more particularly, to a method and composition for increasing the cure depth of such coatings at a low energy level.
2. Description of the Prior Art
Sullivan, in U.S. Pat. No. 4,506,004, describes a contact method of imaging a liquid solder mask coating on a printed circuit board. In this method, the UV curable solder mask coating is screen printed to a given thickness, prehardened and imaged by exposure with suitable UV energy. The desired characteristics of good electrical performance and excellent printing resolution are given as the attributes of the Sullivan system. The requirements of a suitable photoability polymerizable compound for use in this process include an ability to form a smooth, flexible coating which can impart chemical and heat resistance to the solder mask. Furthermore, the coating must exhibit excellent adhesion to the board, and a singular ability to be cured to a desired depth with a minimum of UV energy.
A number of U.S. patents disclose compositions photopolymerizable prepolymers, photoinitiators and additives for making solder masks for printed circuit boards, including U.S. Pat. Nos. 3,660,088; 3,753,720; 3,883,352; 3,887,450: 4,199,163; 4,361,640; 4,458,007; and 4,508,916. However, none of these compositions are entirely satisfactory for use as a commercial solder mask, particularly as a solder mask in the liquid contact imaging system of U.S. Pat. No. 4,506,004, which requires a high cure depth of coating at a low UV energy level. Particularly, it is desired to provide additives in this composition which will improve cure depth under defined exposure conditions while keeping the composition stable towards premature gelation.
Accordingly, an object of this invention is to provide a method and composition which will produce a high cure depth solder mask coating, especially when such coating is prepared in the contact printing technique of U.S. Pat. No. 4,506,004.
Another object herein is to provide a UV cured solder mask coating having a depth of at least 18 mils when exposed to UV light above 300 nm.
Yet another object herein is to provide a stable solder mask composition which provides improved cure depth.
Still another object herein is to provide a method and composition for forming a high resolution solder mask coating of high cured depth at low energy levels.
These and other objects and features of the invention will be made apparent from the following more particular description of the invention.